High-strength nonmagnetic steels



l treated is qui readily or machined, and whichcari" be subse- Patented July 24, 1951 l?ATENT OFFICE HIGH-STRENGTH NONMAGNETIC STEELS Peter Pay so n, NewYork, N. assignor to Crucible Steel Company of America, New York,

N. Y. acorporation of New Jersey No Drawing. Anplication October s, 1949, -SerialNo.120,412

4 Claiins.

This invention pertains to age-hardenable 'austenitic alloy steels, and more particularly to an alloy steel of this type which can be forged androlled,

and which as annealed .or 'solutionte soft, so that it can be cold-formed q'uently age-hardened to a hardness in excess of C 35 Rockwell, and which in the age-hardened condition is characterized in being substantially non-magnetic and in having an 0.02% yield strength of at least 85,000 and for most analyses upwards of 100,000 pounds per square inch.

A broad range of analysis of the steel in ac cordance-with the invention is 'that"--coritaining about: 19 to 25% nickel; 5 to 14% chromium;

.2 to 6%. manganese; 3 to 5% aluminumyfrom' 1 an effective amount up to about 3% silicon; 0.3 to 0.8% carbon; and the balance substantially'iron ex'c'ept'for optional additions of up toabout 3% in aggregate of other elements which donot sube. stantially affect or impair the above mentioned. desirable characteristics of the steel of this invention. As illustrative of optional additions with: .in this category, mention may be made of such,

elements as copper, vanadium, tungsten, molyb-f' denum and cobalt. The usualv impurities, such I as phosphorous, sulphur, etc., may be present inj the steel of the invention .within' commercial .j. tolerances.

A preferred range of cordance with the invention is that containingabout: 20 to 22% nickel; 8 to 11% chromium; 3.5 to5% manganese; 3 to 4% aluminum; 013 to 0.6% carbon; and the balance substantially iron except for impurities within commercial tolerances, and optional minor additions of other elementsflas aforesaid.

The steel of the invention is of especial utility" 'for use in retainer rings on the rotorsof large electric generators and the like; also for use in periscope tubes in submarines, wherein a nonmagnetic steel of high hardness combined with high yield strengthis required.

In this connectionthere has long been a need for, a high strength non-magnetie-material for use as retainer rings on the rotors of large electric generators. is to hold in place on the rotor the heavy conductors which carry the magnetic field circuit... ,The,

centrifugal force generated by the high speed at which the rotor revolves durin the .operationof ,the generator tends to throw these conductorstoff the rotor and it is necessaryto have a strongrei taining ring to keep them in place. For many); machines the retaining rings are made ofa low "alloy steel such as SAE 4140 heat treat'ed tohave a 0.02% ield strength oi at least 100,00 01 ung,s

analysis of steel ill 840;

The function of the retainer ring per square inch p. s. i.) These provide adequate strength for the application, but they seriously limit the efficiency of the generators in which they are. used by the fact that they are magnetic. Retaining rings, made of magnetic material have eddy currents induced in them because they are in close contactwith conductors carrying large currents. These eddy currents not only represent wasted energy in the system, but also generate heat in the rings which causes undesirable operating conditions. If the rings are non-magnetic, the energy loss is negligible and the rings remain H quite cool.

It is well known'that steel can be non-magnetic if the composition is so balanced that austenite, thenon-magnetic phase of iron, is retained at room temperature. The well known 18-8, type 302 Stainless, is an example of this. However,

this steel and all other austenitic steels have rela- 20 tively low yield strengths unless they are severely a cold-worked. When some austenitic steels such astype 302 are severely cold-worked they become magnetic because the cold-working induces the 1. phase change from austenite to ferrite. How- 'ever,[.there are some austenitic steels which are sufiiciently stable so that they remain austenitic, thatis, non-magnetic even after they have been severely cold-worked. The 25 Cr-20 Ni steel, type 3'1O Stain1ess, is an example of such a steel. An-

so: other example is a steel containing I o Mn Si Ni Cr Mo which has been used in the electrical industry for ,non magnetic retaining rings. The short-coming of,thisinaterialhowever is that in the forging of the rings made from it, temperatures and amounts of forging have to be very carefully controlled so I that the cold working may be of exactly the right *"amount to develop in the steel the very high yield strength required of these retaining rings. The electrical industry would much prefer a material in which the high yield strength could be developed by heat treatment rather than by carefully controlled cold-working.

:I-Ieat treatment in the ordinary sense applied to steels, that is, a heating to form austenite; followed by a cooling to permit the austenite to transform to martensite or other low temperature i transformation product; followed by a tempering treatment to stress relieve and toughen the steel; 55, cannot be considered for non-magnetic steel because all transformation products of austenite ,f fsuelif as pearlite, bainite, and martensite are 3 magnetic. For this application it is desirable that the steel be hardened by heat treatment Without causing the austenite to transform.

In my..copendingrapplicationSeriaLNo. J7 92,929, filed December ZO, 1%7, I have :shown thataau'stenitic steels can be hardened by heat treatment if the composition is such that precipitation hardening occurs, When the steel is given suitable solution and aging treatments. 'Sucha precipitation hardening steel as described .in .my-said. copending application has the following constituentsand range of analysis:

Carbon to 0.4% Manganese plus nickel About 20 to 30% Chromium About to 25 Aluminum About 2E5"to"4i5% Molybdenum and/or :tungsten 'Upztoaboutb5% LBalance substantiallyziron The above analysis was developed'to'provide a 'hard'enable corrosion and heat-resistingausten- ,itic steel. In thesteel ofthe present application, the :corrosion and heat resistant features are of .nosignificance," the main requirements beinghigh yield strength, low permeability, and "low .cos't. 'Tdkeepthe cost 'down'itis desirable .to'use as littlealloy as possibleandthis has been 'accom- 1.0 invention specimens were solution treated at 1230.0" fforabout I0 minutes, water quenched (and .themagectat 111002 1200", and 1300 F. for 16 hours, and tested for hardness. The heat "treated pieces were also tested for relative maglfi netism by measuring the pull in grams on a magnetichalance.

I have discovered that both manganese and chromium and also carboninsuitable amounts .are very efiectivein raising th attainablelhard- 20'ness 'o'f 'austenitic, non-magnetic nickel-aluniinum steels to overRockwell C 35. This-was unexpected since ithad'been assumed that .precip'itation hardening in this type of .austenitic .steel was dependent on a nickel-.aluminumeompoun'd, andthat thebalance .o'f the-composition "was relatively unimportant. .Thedata showing thelefiectsof manganese and .chromiumare given lniTablesi'Irand'II.

TABLE I Effect :ojtmanganeseon thewhardening offausten- "[Samples solutinnitreateiat2300 T 'water quenchedand age'd for- 16 hours atin'diz'catedtemperatures] .amlysigTerfCefit Nora: All steels'practiclly nonmagnetic as hardened.

plishedin the "present invention 'rbyilimiting the nickel to about 225% maximum; limiting the chromium to about 11m, maximum;ikeeping the need for expensive low-carbon ferroalloys andto -=permit a "freer use "of scrap steel; omitting molybdenum"and tungsteng-and depending onthe balance of carbon, manganeseniickel, chromium,

lIt'iis clear in'both these groups that steels with :manganese under 1.5% donot harden .as .welLas those with m'anganese from about"2%-.up.to.about carbon.over.about.0.30%, thus to.minimize the 75%. I".[t.also 'appears'from the tests on B'ar'5816 thatmanganese over 6% is detrimental. .Aecord- .ingly I set the limits .onmanganese in .the steel of this? invention atapproximatelylfrom 2Z0 to 6.'0.%, "with 'thepreferredrange at 4.0 .to 5.0%

TABLE II jfietrt 0') chromium 0n the hardening 10f ausifSamples solutinnztreate'iiz'at iisoo" F.,'water'quenched and aged for 16 hoursat iindicatedtemperaturea] N91 11: steels practically non-magnetic as hardened.

[Samples solution treeted at 2300" F., water quenched and aged f or 16 116111333 The data of Table II' show that a minimum of about 5% chromium is necessary for adequate hardening of this steel, and that the hardnessga-z"v tends to become constant or to decrease somewhat as the chromium is increased beyond about 8%. Accordingly I set the limits on chromiumin the steel of this invention at from 5 to 14%, with the preferred range at 8 to 11%, thelatter, I

for optimum hardening.

10 that the steels of higher carbon havehigher This is confirmed by the following It was indicated to a slight degree in Table I hardness. data of Table III.

TABLE HI ing and that hardenin improves rapidly with increasing aluminum. However, the steel becomes difficult; to forge as the aluminum is increased beyond about5%. Fur thermore,because of its strong ferrite forming tendencies, aluminum in increasing amount tends to make the steel ma netic. I therefore set the limits for aluminum in the steel of this invention at about 3.0- to 5.0% with a preferred range of 3.0 to 4.0%. 1

,It is desirable from the standpoint of econ'omy to hold thenikel content of the steel to a minimum. However, as shown in Table V, it is necessary to have a minimum of about 19 Efiect of canbrm on the hardening} of austenitic 5% Mn-21% Ni-8.5% C'r-3% AZ steel cated temperatures] Analysis, Per Cent Rockwell o Hardness After Aging I Norm: all steels practically non-magnetic as hardened.

This shows that a minimum of about 0.30% 30 carbon is needed in this steel for adequate harden'ing' and that hardness increases with increasing carbon; However, since carbon beyond about 210.80% would make the steel verydifiicult to forge from Table IV.

'nickelin' the steel of this invention in order to assure adequate hardening and also to'ass'ure stability of the austenite so that it does not transform to a magnetic constituent'during the "aging treatment;

. .7TAB1EV Efiectmof -nickel on the hardening of austenitic.

- Mn-.9% C'r- 3.5% Alsteel m [Samples solution treatedat-ZSOO? F., water quenched and aged for 16 hours at indicated temperatures.

. 1 RoclrwellGHardnessAfter Analysis, Per Cent Aging 'fo'" 81 N1 '01 Al 11o0=r. 1200 1. 1300F.

N OTE: The 17.5% Ni steel was slightly magnetic as aged at 1200 F. and stronsly magnetic after the 1300 F. aging. the 1300' F, aging.. 1 All other samples were practically non-magnetic.

I set the carbon limits of the steel of this inven'-,,

tion at 0.30 te 0.80% with a preferred range of Aluminum has an appreciable effect on the *hardening of the austenitic steel as may be seen.

TABLE IV The 19.4% Ni steel was slightly magnetic a1 er 0n the basis of these data the limitsof nickel in the steel of this invention are set at 19 to 25 with a preferred range at 20 to 22 l The silicon content of the steel within the limits ordinarily found in forgeable steelsseems Efi'ectof'aZuminum on the hardening) of adsi 'tem'tiq 4% Mn-21% Ni-9% CT Steel E [Samples solution treated at 2300 F., water quenched and aged for 16 hours at indicated temperatures] Analysis,Per c9111; WQAggfiEEQWf" 1 .Bar 4. 2

0 Mn Si Ni 01', .41 1100 11; 1200F.- -1300-F.i'"- 1' .43- 3.02 .00 21.36 3.57 2.61 s 26 1 27:. .40 3.84 .68 21.26 8.68 v3.46 17 ..45; 35 l,

Nora: The 3.8% aluminum steel was very slightly magnetic when aged at F I All other samples-were practically non-magnctlc. 1

Thus it may be seen that a minimum of about 3% aluminum is required for-adequate harden- "to'have only a minor effect on the hardening of r the steel as shown'in' Table VI. w-am.

users-n TABLE .VI

Efieet of silicon on the hardening of austenitic Mn-Zl Ni-8% CT-13% Al'steel NOTE; All steels practically non-magnetic as hardened.

The limits for silicon in the steel of this .invention are therefore set at from an effective amount to about 3.0%.

Aside from those elements already specified above, the balance of the steel consists of iron alone, or with up to about 3% in aggregate .of.

strength-are attainable in the steel of this invention with asolution treatment of about .2300" F. and an agingat about 1200 F., a good value-of yield strength can be obtained together with a low value of permeability with is. solution treatment at 2200 F., and even at 2100 F. as shown in Table VII. Furthermore, better values of ductility are obtainablebyuse of the lower solution temperature.

TABLE VII bon; up to about 3% in aggregate of .other elements which do not substantially impair the age-hardening and non-magnetic properties of the steel; and the balance substantially all iron.

2. A forgeable and machineable, age-hardenable, .austenitic alloy steel which, on solutiontreating at about 2100 to 2300 F., followedby aging at about 1200 to 1300 F., is characterized in being substantially non-magnetic and in having a hardness in excess of Rockwell C 35-andan 0.02% yield strength of at'least'85;000 pounds per square inch, said steel containing: about 20 to 22% nickel; about-8 to 11 chromium; about 3.5 to 5% manganese; about *3 to 4% aluminum; about 0.3 to 0.6% carbon; up to about 3% in aggregate of other-elements which do -not substantially impair the iageehardening and. non-magnetic properties of the steel; and the balance substantially all iron.

3. An age-hardened, austenitic alloy steel which is characterized in being substantially Tensile and permeability values pf aye hardened austeniticMn-Ni-Cr-Al steel mechanical and m agneticproperties] {Samples solutiontreatedas indicated and-aged for '161201115'815 12002 F., "then tested for room temperature Tens. E1. in 2 Permea- Sol. Book. R. A. Bar 0 Mn .Ni Or Al Strength, Strength, .in., Per bility at C .DJS. i. p.'s.i. Cent Per Cent H=l000 5880... 44 3. 0 21. 5 8. 7 '3. 7 2,200 41 161,000 95, 000 15. 0 35. 4 1. 033 "5880.-- 44 '3. 0 .21. 5 8. 7 3. 7 2,300 45 173, 000 .121, 000 6. 0 15. 2 5815 35 3. 8 21. 8 8. 8 '3. 6 2, 300 43 221, 000 104, 000 6. 4 19. 1 1. 008 5881. .49 3. 8 21. 3 8. 7 -3. 5 2, 200 37 150, 000 85, 000 21. 2 33.1 {5851... 49 3. 8 21.3 8.7 3.5 2, 300 39' 179, 000 119, 000 v 4.0 13.6 -5882 46 4. 7 21. 3 8.7 3. '6 2, 100 '37 178.000 97,000 19. 2 25. 5 1. 015 5882. 46 4. 7 21. 3 8. 7 -3. 6 2,200 38 176, 000 102, 000 13. 3 21. 0 1. 010 5882, ..46 4. 7 21.3 S. 7 3.6 2, 300 ,44 177,000 125, 000 10. 4 2. 4 5818 .34 I 3. 8 '21. l 10. 7 '3; 4 '2, 300 '38 181, 000 105, 000 T6. 7 14. 0 1.006

It will thus be seen that the steel of the invention is characterized in possessing in the ageor precipitation-hardened condition, high hardness combined with high tensile and yield strength and is, moreover substantially nonmagnetic.

I claim:

1. A forgeable and machineable, age-hardenable, austenitic alloy steel which, on solutiontreating at about 2100 to 2300 F., followed by aging at about 1200 to 1300 F., is character-.-

ized in being substantially non-magnetic and in having a hardness in excess of Rockwell C 35 and an 0.02% yield strength of at least 85,000

pounds per square inch, said steel containing;

about 19 to 25% nickel; about 2 to 6% manganese; about 5 to 14% chromium; about 3 to .5%--aluminum; from an efiective amount up to about 3% silicon; from about 0.3 to 033% carnon-magnetic and in having a hardness in excess of about Rockwell C 35 and an 0.02%

, yield strength .of at least 85,000 pounds per square inch, said steel containing: about 19 to 25% nickel; about 2 to 6% manganese; about 5 to 14% chromium; about 3 to 5% aluminum;

from an effective amount up to about 3% sili- .con; from about 0.3 to 0.8% carbon; up to about :3.% in aggregate of other elements *which do not substantially impair the age-hardening and non-magnetic properties of the steel; and the balance substantially all iron.

square inch, said steel containing: about 20 to 22% nickel; about .8 to 11% chromium; about 3 to 5% manganese; about 3 to 4% aluminum; aboutv0.3 to 0.6% carbon; up to about 3% in aggregate of other elements which do not substantially impair the age-hardening and nonmagnetic properties of the steel; and the bal- 5 ance substantially all iron.

PETER PAYSON.

REFERENCES CITED The following references are of record in the 10 I file of this patent:

Number I Number 0 e UNITED STATES PATENTS Name Date Evans Aug. 23, 1927 Pilling July 21, 1936 FOREIGN PATENTS Country Date Great Britain Apr. 30, 1934 

1. A FORGEABLE AND MACHINEABLE, AGE-HARDENABLE, AUSTENITIC ALLOY STEEL WHICH, ON SOLUTIONTEATING AT ABOUT 2100* TO 2300* F., FOLLOWED BY AGING AT ABOUT 1200* TO 1300* F., IS CHARACTERIZED IN BEING SUBSTANTIALLY NON-MAGNETIC AND IN HAVING A HARDNESS IN EXCESS OF ROCKWELL "C" 35 AND AN 0.02% YIELD STRENGTH OF AT LEAST 85,000 POUNDS PER SQUARE INCH, SAID STEEL CONTAINING: ABOUT 19 TO 25% NICKEL; ABOUT 2 TO 6% MANGANESE; ABOUT 5 TO 14% CHROMIUM; ABOUT 3 TO 5% ALUMINUM; FROM AN EFFECTIVE AMOUNT UP TO ABOUT 3% SILICON; FROM AN EFFECTIVE AMOUNT UP TO BON; UP TO ABOUT 3% IN AGGREGATE OF OTHER ELEMENTS WHICH DO NOT SUBSTANTIALLY IMPAIR THE AGE-HARDENING AND NON-MAGNETIC PROPERTIES OF THE STEEL; AND THE BALANCE SUBSTANTIALLY ALL IRON. 